This invention relates to a rotational angle-measurement apparatus using magneto-resistance elements (hereafter referred to also as MR elements).
Such a rotational angle-measurement apparatus as using MR elements is disclosed in, for example, JP-3799270.
As MR elements are known anisotropic magneto-resistance elements (hereafter referred to as AMR elements) and giant magneto-resistance elements (hereafter referred to as GMR elements). The general idea of the related, conventional art will be described below with a magnetic field detection apparatus using GMR elements taken as an example.
FIG. 2 shows the fundamental structure of a GMR element. The GMR element comprises a first magnetic layer (pinned magnetic layer), a second magnetic layer (free magnetic layer) and a non-magnetic layer (spacer layer) interposed between the first and second magnetic layers. When the GMR element is placed in an external magnetic field, the magnetization direction 20 in the free magnetic layer changes depending on the orientation of the external magnetic field while the magnetization direction in the pinned magnetic layer remains unchanged.
When a voltage is applied across the GMR element, current flows in accordance with the element resistance. The element resistance varies depending on the difference Δθ=θf−θp between the magnetization direction θp of the pinned magnetic layer and the magnetization direction θf of the free magnetic layer. Accordingly, if the magnetization direction θp of the pinned magnetic layer is previously known, the magnetization direction θf of the free magnetic layer, i.e. the orientation of the external magnetic field, can be detected by measuring the resistance of the GMR element and using the above difference relationship.
The mechanism of the resistance of the GMR element changing according to the relationship Δθ=θf−θp is as follows.
The magnetization direction in a thin-film magnetic film is related to the direction of electron spin in the magnetic film. Accordingly, if Δθ=0, the spin direction of a majority of electrons in the free magnetic layer tends to coincide with the spin direction of a majority of electrons in the pinned magnetic layer. On the other hand, if Δθ=180°, the spin direction of a majority of electrons in the free magnetic layer tends to be opposed to the spin direction of a majority of electrons in the pinned magnetic layer.
FIGS. 3A and 3B schematically show in cross-section a free magnetic layer 11, a spacer layer 12 and a pinned magnetic layer 13. Arrows in the free magnetic layer 11 and the pinned magnetic layer 13 indicate the spin directions of the majority electrons. FIG. 3A shows the case where Δθ=0, that is, the spin direction of the free magnetic layer 11 coincides with that of the pinned magnetic layer 13. FIG. 3B shows the case where Δθ=180°, that is, the spin direction of the free magnetic layer 11 is opposite to that of the pinned magnetic layer 13. In case of Δθ=0, as shown in FIG. 3A, electrons having the spin direction to the right, issued from the pinned magnetic layer 13, are scattered less frequently in the free magnetic layer 11 whose majority electrons have the spin direction to the right, traveling along such a path as an electron trajectory 810. On the other hand, in case of Δθ=180°, as shown in FIG. 3B, electrons having the spin direction to the right, issued from the pinned magnetic layer 13, are scattered more frequently in the free magnetic layer 11 whose majority electrons have the spin direction to the left, traveling along such a path as an electron trajectory 810. In this way, if Δθ=180°, electron scattering is considerable so that the electric resistance in the GMR element increases.
In the intermediate case of Δθ=0˜180°, the electron trajectory becomes somewhat intermediate between those shown in FIGS. 3A and 3B. The resistance of the GMR element is known to be represented by the following expression.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                            R        =                                            R              0              ′                        +                                          G                2                            ⁢                              (                                  1                  -                                      cos                    ⁢                                                                                  ⁢                    Δθ                                                  )                                              =                                    R              0                        -                                          G                2                            ⁢              cos              ⁢                                                          ⁢              Δθ                                                          (        1        )            
Here, G/R is called the GMR coefficient, having a value of several to several tens of percent.
As described above, since electric current (therefore, electric resistance) through the GMR element can be controlled depending on the direction of electron spin, the GMR element is also called a spin-valve device.
Moreover, with a magnetic film having a small film thickness (thin-film magnetic films), since the demagnetizing factor in the normal direction with respect to the surface is extremely large, the magnetization vector cannot rise up in the normal direction (direction of film thickness) and remains recumbent in the plane of the surface. Since each of the free magnetic layer 11 and the pinned magnetic layer 13, which constitute the GMR element, is sufficiently thin so that the magnetization vectors of the layers 11 and 13 lie in their planes.
In a magnetic field detection apparatus, four GMR elements R1 (51-1)˜R4 (51-4) constitute a Wheatstone bridge as shown in FIG. 4. Here, let it be assumed that the magnetization direction in the pinned magnetic layers of the GNR elements R1(51-1) and R3(51-3) is given by θp=0 while that of the GNR elements R2 (51-2) and R4 (51-4) is given by θp=180°. Since the magnetization directions in the free magnetic layers of the four GMR elements are determined depending on the orientation of the external magnetic field, they become the same as one another. It therefore holds that Δθ2=θf−θp2=θf−θp1−π=Δθ1+π. Here, since Δθ1 is set with θp=0 as reference, the replacement Δθ1=0 is introduced. Accordingly, as apparent from the expression (1), it follows that for R1 and R3 (n=1, 3):
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                      R          n                =                              R                          n              ⁢                                                          ⁢              0                                +                                    G              2                        ⁢                          (                              1                -                                  cos                  ⁢                                                                          ⁢                  θ                                            )                                                          (        2        )            
And it follows that for R2 and R4 (n=2, 4):
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      R          n                =                              R                          n              ⁢                                                          ⁢              0                                +                                    G              2                        ⁢                          (                              1                +                                  cos                  ⁢                                                                          ⁢                  θ                                            )                                                          (        3        )            
When an excitation voltage e0 is applied to the bridge shown in FIG. 4, the difference voltage ΔV=V2−V1 between the terminals 1 and 2 is given by the following expression.
                    [                  Expression          ⁢                                          ⁢          4                ]                                                                      Δ          ⁢                                          ⁢          v                =                                                                              R                  1                                ⁢                                  R                  3                                            -                                                R                  2                                ⁢                                  R                  4                                                                                    (                                                      R                    1                                    +                                      R                    4                                                  )                            ⁢                              (                                                      R                    2                                    +                                      R                    3                                                  )                                              ⁢                      e            0                                              (        4        )            
If the expressions (2) and (3) are substituted for the expression (4), if it is assumed that Rn0's are equal to one another for n=1˜4, and that R0=Rn0, then it follows that:
                    [                  Expression          ⁢                                          ⁢          5                ]                                                                      Δ          ⁢                                          ⁢          v                =                                            -                              e                0                                      ⁢            G            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                                2            ⁢                          R              0                                                          (        5        )            
In this way, since the signal voltage ΔV is proportional to cos θ, the orientation of magnetic field can be detected.
Thus, the magneto-resistance element is characterized in that it can directly measure the orientation of magnetic field.
There is known a rotational angle-measurement apparatus using a resolver and such is disclosed in JP-A-2008-11661. As disclosed in JP-A-2008-11661, the resolver measures the change in the inductance along the closed path: stator coil˜rotor core˜stator coil. By appropriately designing the shape of the rotor core, the length of the air gap between the rotor core and the stator can be made variable depending on the rotational angle of the rotor. Hence, the inductance changes accordingly. Therefore, the rotational angle of the rotor core can be measured by measuring the change in the inductance.
In this way, with a rotational angle sensor the typical example of which is a resolver, that measures inductance, the accuracy of air gap affects the accuracy in angle measurement so that high accuracy is required in fabrication and assembling. Moreover, increase in the diameter of the rotor shaft causes increase in the size of the resolver. This results in a problem of cost increase.
On the other hand, the size of a magneto-resistance element such as a GMR element is a square having its side of a few millimeters or less. It therefore can be said to be of small size and light weight. Moreover, since the magneto-resistance element detects the orientation of magnetic field, a small-sized sensor can be used even if a thick rotor shaft is used.
Accordingly, if it is desired to build a small-sized rotational angle-measurement apparatus, the use of magneto-resistance elements can advantageously provide a desirable apparatus of smaller size and lighter weight. Further, if it is desired to control an electric motor of large rating, the use of magneto-resistance elements can advantageously provide a low-cost rotational angle-measurement apparatus.